getMethod {methods}

R Documentation

Get the Definition of a Method

Description

A call to getMethod returns the method corresponding a particular generic function and signature; a call to findMethod returns the vector of package names in which a matching method is found.

The functions MethodsListSelect and selectMethod select methods using an environment in which arguments to the function have been assigned (they differ in the treatment of inheritance). Neither is intended to be called directly, except perhaps in debugging. The function getMethods returns all the methods for the generic function.

The function findMethod returns the package(s) in the search list (or in the packages specified by the where argument) that contain a method for this function and signature.

Usage

getMethod(f, signature=character(), where, optional=FALSE)

findMethod(f, signature, where)

getMethods(f, where=-1)

MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs, useInherited)
selectMethod(f, signature, optional=FALSE, useInherited, mlist=getMethods(f))

Arguments

f	The character-string name of the generic function. In getMethods only, this argument may be a function definition, in which case the special methods list object, if any, stored in the environment of the function is returned. (This usage is largely for internal purposes; you aren't likely to have such a function definition for direct use.)
signature	The signature of classes to match to the arguments of f. The vector of strings for the classes should be named, and the names must match formal argument names of f. If not named, the signature is assumed to apply to the arguments of f in order, but note below for selectMethod. For selectMethod, the signature can optionally be an environment with classes assigned to the names of the corresponding arguments. If the signature is not an environment, it must currently be named.
where	The position or environment in which to look for the method: by default, anywhere in the current search list.
optional	If the selection does not produce a unique result, an error is generated, unless this argument is TRUE. In that case, the value returned is either a MethodsList object, if more than one method matches this signature, or NULL if no method matches.
mlist	In selectMethod, the MethodsList object can be explicitly supplied. (Unlikely to be used, except in the recursive call that finds matches to more than one argument.)
env	The environment in which argument evaluations are done in MethodsListSelect. Currently must be supplied, but should usually be sys.frame(sys.parent()) when calling the function explicitly for debugging purposes.
fEnv, finalDefault, evalArgs, useInherited	Internal-use arguments for the function's environment, the method to use as the overall default, whether to evaluate arguments, and which arguments should use inheritance.

Details

A call to getMethod returns the method for a particular function and signature. As with other get functions, argument where controls where the function looks (by default anywhere in the search list) and argument optional controls whether the function returns NULL or generates an error if the method is not found. The search for the method makes no use of inheritance.

The function getMethods returns all the methods for a particular generic (in the form of a generic function with the methods information in its environment). The function is called from the evaluator to merge method information, and is not intended to be called directly.

The function MethodsListSelect performs a full search (including all inheritance and group generic information: see the Methods documentation page for details on how this works). The call returns a possibly revised methods list object, incorporating any method found as part of the allMethods slot.

Normally you won't call MethodsListSelect directly, but it is possible to use it for debugging purposes (only for distinctly advanced users!).

Note that the statement that MethodsListSelect corresponds to the selection done by the evaluator is a fact, not an assertion, in the sense that the evaluator code constructs and executes a call to MethodsListSelect when it does not already have a cached method for this generic function and signature. (The value returned is stored by the evaluator so that the search is not required next time.)

The selectMethod function is similar to MethodsListSelect (and calls it) but differs in that it is intended to be used with control over which arguments are allowed to use inheritance. The restriction is needed to make sensible method selection for coerce methods (the as function).

Value

The call to selectMethod or getMethod returns a function object, the selected method, if a unique selection exists. Otherwise an error is thrown if optional is FALSE. If optional is TRUE, the value returned is NULL if no method matched, or a MethodsList object if multiple methods matched.

The call to getMethods returns the MethodsList object containing all the methods requested. If there are none, NULL is returned: getMethods does not generate an error in this case.

Author(s)

John Chambers

References

The web page http://www.omegahat.org/RSMethods/index.html is the primary documentation.

The functions in this package implement a facility for classes and methods as described in Programming with Data (John M. Chambers, Springer, 1998). See this book for further details and examples.

---